Little research has been conducted on the heterogeneous kinetics of coal oxidation in supercritical water (SCW). Therefore, the oxidation of coal particles in SCW coupled with heat and mass transfer was investigated. The real coal particle temperature was estimated in terms of a simplified heat balance. With increasing particle temperature, the rate-limiting step of the oxidation reaction gradually changed from the surface reaction to the mass transfer of oxygen. According to this result, the reaction rate model was built. The model could be used to predict the quantitative relationship among the particle size, the particle temperature, and the time to complete oxidation.